(a) Field of the Invention
The present invention relates to a positive active material for a lithium battery, a method of preparing the same, and a rechargeable lithium battery including the same. More particularly, the present invention relates to a positive active material having excellent high-capacity and thermal stability, a method of preparing the same, and a rechargeable lithium battery including the same.
(b) Description of the Related Art
Lithium ion rechargeable batteries have been widely used as power sources for portable electronic devices since 1991. Recently, portable electronic devices (e.g., camcorders, cell phones, laptop computers, and so on) have been markedly developed with the rapid development of electronic, communication, and computer industries. Therefore, the lithium ion rechargeable batteries are required to supply the power for these portable electronic devices. In particular, a power source for a hybrid electric vehicle having both an internal combustion engine and a lithium rechargeable battery is being actively researched in the United States, Japan, Europe, and so on.
A high-capacity battery for an electric vehicle is the initial stage of development. Generally, nickel hydrogen batteries are used due to its safety, but the lithium ion batteries are advantageous in terms of energy density. However, the lithium ion batteries still have problems of both a high price and safety to be solved. Particularly, both LiCoO2 and LiNiO2 positive active materials that are commercially available have an unstable crystal structure due to delithiation upon charging the battery, so thermal characteristics of the LiCoO2 and LiNiO2 positive active materials are very deteriorated. That is, if the overcharged battery is heated at a temperature of 200° C. to 270° C., the structure of the battery is rapidly changed to cause an oxygen emitting reaction in lattices of the changed structure (J. R. Dahn et al., Solid State Ionics, 69, 265, 1994).
Concurrently, commercially available small lithium ion rechargeable batteries generally include LiCoO2 used as a positive active material. LiCoO2 is a material having stable charge and discharge characteristics, excellent electron conductivity, high stability, and a smooth discharge voltage characteristic. However, since cobalt (Co) is a rare material and has a high cost and toxicity to people, new positive electrode materials capable of replacing Co are required. Although LiNiO2 having a layered structure similar to LiCoO2 has a high discharge capacity, it has not commercially developed due to its unstable thermal and lifetime characteristics as well as its lack of safety at a high temperature. To solve these problems, it has been attempted to substitute a portion of nickel with transition metal elements so that the exothermic temperature is increased and to make a broad exothermal peak such that a rapid exothermal reaction is inhibited (T. Ohzuku et al., J. Electrochem. Soc., 142, 4033, 1995, No. 9-237631). However, the results have not yet been confirmed.
In addition, LiNi1−xCoxO2 (x=0.1-0.3) materials in which a portion of nickel is substituted with thermally stable cobalt shows good charge-discharge and lifetime characteristics, but it cannot provide thermal stability. Moreover, there have been suggested a Li—Ni—Mn-based composite oxide in which a portion of Ni is substituted with thermally stable Mn and a Li—Ni—Mn—Co-based composite oxide in which a portion of Ni is substituted with Mn and Co and methods of preparing the same.
For example, Japanese Patent laid-open Publication Hei 08-171910 discloses a method of preparing a positive active material of LiNixMn1−xO2 (0.7≤x≤0.95) including: mixing an aqueous solution including Mn-containing salt and Ni-containing salt with an alkaline solution to co-precipitate Mn and Ni; mixing the co-precipitated compound with a lithium hydroxide; and firing the mixture of the co-precipitated compound and the lithium hydroxide.
Recently, Japanese Patent laid-open Publication No. 2000-227858 disclosed a positive active material in which Mn and Ni compounds were uniformly distributed at an atomic level to provide a solid solution instead of the concept that a transition metal element is partially substituted into LiNiO2 or LiMnO2.
According to European Patent No. 0918041 or U.S. Pat. No. 6,040,090, LiNi1−xCoxMnyO2 (0≤y≤0.3) has improved thermal stability compared to that of materials composed of only Ni and Co. However, LiNi1−xCoxMnyO2 (0≤y≤0.3) may not be commercially developed due to its reactivity with an electrolytic solution of Ni4+. In addition, European Patent No. 0872450 A1 discloses LiaCobMncMdNi1−(b+c+d)O2 (M=B, Al, Si, Fe, Cr, Cu, Zn, W, Ti, or Ga) in which Ni was substituted with another metal as well as Co and Mn. However, since the active materials disclosed in these patents still include Ni, the thermal stability of the active materials is not fully improved.
The most spotlighted materials that have a layered crystal structure and is capable of replacing LiCoO2 may include Li[Ni1/2Mn1/2]O2 of which nickel and manganese are mixed at a ratio of 1:1 and Li[Ni1/3Co1/3Mn1/3]O2 of which nickel, cobalt, and manganese are mixed at a ratio of 1:1:1. These materials have advantages of lower cost, higher capacity, and superior thermal stability than LiCoO2. However, the materials have lower electron conductivity than LiCoO2, so high rate capacity and low temperature characteristics of the materials are deteriorated. In addition, even though the capacities of the materials are higher than that of LiCoO2, the energy density of the battery including the same is not improved due to its low tap density. In particular, since these materials have low electronic conductivity (J. of Power Sources, 112, 2002, 41-48), their high power characteristics are inferior to that of LiCoO2 or LiMn2O4 when used in a hybrid power source for electric vehicles.
Li[Ni1/2Mn1/2]O2 and Li[Ni1/3Co1/3Mn1/3]O2 can be prepared by simultaneously precipitating two or three elements in an aqueous solution using a neutralization reaction to form a hydroxide or an oxide precursor, mixing the precursor with lithium hydroxide, and firing the same. Unlike the general co-precipitation reaction, a co-precipitated particle including manganese is shaped as an irregular plate and has a half tap density comparable to that of nickel or cobalt. For example, according to Japanese Patent laid-open Publication No. 2002-201028, a conventional reactor was used by the inert precipitation method, the generated precipitate particles were widely distributed, and shapes of primary particles were different from each other. In addition, Japanese Patent laid-open Publication Nos. 2003-238165, 2003-203633, 2003-242976, 2003-197256, 2003-86182, 2003-68299, and 2003-59490 and Korean Patent Nos. 0557240 and 0548988 disclose a method of preparing a high-capacity positive active material capable of improving charge and discharge reversibility and thermal stability by dissolving a nickel salt and a manganese salt, or a nickel salt, a manganese salt, and a cobalt salt in an aqueous solution, simultaneously introducing an alkali solution into a reactor while introducing a reductant or an inert gas to obtain a metal hydroxide or an oxide precursor, mixing the precursor with lithium hydroxide, and firing the same.
As described above, lithium transition metal-based oxides having a R m layered crystal structure includes LiCoO2, LiNiO2, LiNi1−xCoxO2, LiNi1−x−yCoxMyO2 (M=Mn, Al, Mg, Ti, Ti1/2Mg1/2), LiNi1/3CO1/3Mn1/3O2, LiNi1/2Mn1/2O2, LiNixCO1−2xMnxO2, and Li1+z[NixCO1−2xMnx]1−zO2. Generally, these materials have a uniform metal composition on surfaces of particles and in their bulks.
To provide an excellent performance of the positive electrode, functions acting on the inside and the surface of a positive electrode power particle should be different from each other. In other words, the composition of the inside of the particles should have a lot of spaces which lithium ions are intercalated in or separated from and the particles should structurally stable. In addition, the reactivity of the surfaces of the particles with the electrolytic solution should be minimized.
A surface treatment method is used as a method of changing the surface composition of the positive active material. The surface treatment method includes coating a nanometer-thin coating layer on surfaces of the powder particles with a coating amount of 1 wt % to 2 wt % with respect to the total weight of the positive active material to inhibit the reactivity with the electrolytic solution; and heating the nanometer-thin coating layer to form a solid solution on the surfaces of the powder particles. Since the solid solution is formed on the surfaces of the powder particles, the metal composition of the surfaces of the powder particles is different that of the inside of the particles (J. Cho et al., J. Electrochem. Soc., 149, A127, 200; J. Dahn et al., Electrochem. and Solid State Lett., 6, A221 2003, U.S. Pat. Nos. 6,555,269, 6,274,273). In the event that the coating layer is formed on the surfaces of the powder particles by coating and heating, the surface layer combined with the coating material has a thickness of several tens or less nanometers and the composition ratio of the coating layer is different from that of the bulk of the particle. Thus, a coating efficiency may be reduced when the batteries are repeatedly used hundreds times. In addition, the coating layer is not uniformly distributed on the surfaces so that the coating efficiency is also reduced.
To overcome these problems, Korean Patent Laid-open Publication No. 2005-0083869 disclosed a lithium transition metal oxide having a concentration gradient of the metal composition. According to this method, after an inner material is synthesized, a material having another composition ratio is formed on the inner material to fabricate a double layer. The double layer is mixed with lithium salt and the mixture of the double layer and the lithium salt is heated. The inner material may be used as a commercially available lithium transition metal oxide.
According to this method, the metal compositions of the inner layer and the outer layer may be different from each other but the metal composition of the generated positive active material is not continuously and gradually varied. The metal composition of the positive active material may have a gradual gradient by a heat treatment process, but a concentration gradient difference of the positive active material hardly occurs by thermal diffusion of metal ions caused at a temperature of 850° C. or more. In addition, since the powder synthesized by the patent does not use ammonia corresponding to the chelating agent, a tap density of the powder is low to be unsuitable for a positive active material of a lithium rechargeable battery. Furthermore, according to the method, it is hard to control the lithium amount of the outer layer when the lithium transition metal oxide is used as the inner material, so the reproducibility is deteriorated.
Japanese Patent No. 2002-001724 discloses a composite positive active material formed by mixing a high-stable composite oxide Li1.02Ni0.65Mn0.35O2 with a high-conductivity composite oxide Li1.02Ni0.7Co0.3O2. The high-stable composite oxide Li1.02Ni0.65Mn0.35O2 has an excellent lifetime characteristic and excellent thermal stability but has deteriorated conductivity and deteriorated discharge capacity. The high-conductivity composite oxide Li1.02Ni0.7Co0.3O2 has excellent conductivity and excellent discharge capacity but has a deteriorated lifetime characteristic and deteriorated thermal stability. However, as a composition ratio of the high-stable composite oxide increases, the lifetime characteristic of the composite positive active material was excellent. As a composition ratio of the high-conductivity composite oxide increases, the high rate capacity of the composite positive active material was excellent. In other words, a positive active material with both high rate capacity and excellent lifetime characteristics has not been provided.